Display devices have become thinner and bigger as an industrial utilization increases. Among the various types of flat panel display (FPD) devices, liquid crystal display (LCD) devices and plasma display panel (PDP) devices are widely used.
A PDP device is an emissive type display device where light is emitted from fluorescent materials in a sidewall between two substrates in response to an applied voltage. An LCD device is a non-emissive type display device where images are displayed by adjusting light from a backlight unit with a liquid crystal layer as a light modulator. Since grey levels are displayed by a digital voltage in a PDP device, the PDP device has a disadvantage in displaying natural images. On the contrary, since an analog voltage is applied to both sides of a liquid crystal layer in an LCD device, the LCD device displays a natural image as compared with a PDP device.
Among LCD devices, an active matrix liquid crystal display (AMLCD) device is widely used. In an AMLCD device, a thin film transistor (TFT) is connected to a pixel and adjusts a voltage level of the pixel as a switching element to change light transmittance of the pixel and display images.
In FIG. 1, a liquid crystal display (LCD) device includes a liquid crystal panel 1, a gate driver 4, a data driver 6, a timing controller 7, a backlight unit 8, and a source voltage generator 9. A plurality of thin film transistors (TFTs) are disposed in matrix on the liquid crystal panel 1. The gate driver 4 controls input of a data signal into the liquid crystal panel 1, and the data driver 6 outputs the data signal to the liquid crystal panel 1. The timing controller 7 controls a timing of the gate driver 4 and the data driver 6. The backlight unit 8 is disposed underneath, and supplies light to, the liquid crystal panel 1. The backlight unit 8 includes a backlight lamp 8a emitting the light, and a backlight driver 8b to control the backlight lamp 8a. The source voltage generator 9 supplies source voltages to the gate driver 4, the data driver 6, the timing controller 7 and the backlight unit 8. The source voltage generator 9 is formed on a printed circuit board (PCB). Although not shown in FIG. 1, the backlight lamp 8a includes one of at least one fluorescent lamp or a plurality of light emitting diodes (LEDs).
Each TFT uses hydrogenated amorphous silicon (a-Si:H) for a semiconductor layer. The hydrogenated amorphous silicon yields higher productivity while being easily fabricated on a large sized substrate. In addition, since the hydrogenated amorphous silicon is deposited at a temperature less than about 350° C., a low-cost glass substrate can be used. The hydrogenated amorphous silicon is used mainly in a TFT, which is referred to as an amorphous silicon thin film transistor (a-Si TFT). However, since the hydrogenated amorphous silicon has a disordered atomic arrangement, weak silicon-silicon (Si—Si) bonds and dangling bonds exist in the hydrogenated amorphous silicon. These types of bonds become metastable when light or an electric field is applied to the hydrogenated amorphous silicon. As a result, this metastability makes the TFT unstable. Specifically, the electrical characteristics of the hydrogenated amorphous silicon are degraded due to light irradiation. Furthermore, a TFT using the hydrogenated amorphous silicon is difficult to be implemented in a driving circuit due to degraded electrical characteristics such as a low field-effect mobility between about 0.1 cm2/Vsec to about 1.0 cm2/Vsec, and poor reliability.
The substrate including the a-Si TFT is connected to a printed circuit board (PCB) using a tape carrier package (TCP) that has a driving integrated circuit (IC). The driving IC and its packaging increase production cost of the LCD device. As the resolution of a liquid crystal panel for an LCD device increases, a pad pitch between gate pads or between data pads of the substrate including the a-Si TFT becomes smaller. Thus, bonding of the TCP and the substrate including the a-Si TFT becomes harder.
To solve these problems, a polycrystalline silicon thin film transistor (p-Si TFT) has been suggested. Due to the higher field effect mobility of a p-Si TFT as compared to an a-Si TFT, a driving circuit can be integrated on a substrate including the p-Si TFT such that a driving element and a switching element are simultaneously formed. Accordingly, the TCP is eliminated and the production cost is reduced. Moreover, a driving system may be integrated in the liquid crystal panel. An LCD device where a driving system is integrated in a liquid crystal panel may be referred to a system-on-panel (SOP) type LCD device.
FIG. 2 is a block diagram showing a liquid crystal display device using a polycrystalline silicon thin film transistor. In FIG. 2, a liquid crystal display (LCD) device includes a liquid crystal panel 10. The liquid crystal panel 10 includes two attached substrates (not shown). A display area 12 for displaying images, and a non-display area 13 for driving the display area 12 are defined in the liquid crystal panel 10. A gate line “GL” and a data line “DL” crossing each other are formed in the display area 12. In addition, a thin film transistor (TFT) “T” is connected to the gate line “GL” and the data line “DL.” A gate driver 14 and a data driver 16 are formed in the non-display area 14. The gate driver 14 and the data driver 16 receive a gate signal and a data signal from an exterior system (not shown) and control the TFT “T” in the display area 12 through the gate line “GL” and the data line “DL,” thereby changing light transmittance of a liquid crystal layer between the attached two substrates. Although not shown in FIG. 2, a timing controller and a source voltage generator are formed on a printed circuit board (PCB) and connected to the liquid crystal panel 10. The backlight unit is disposed under the liquid crystal panel 10.
Since a backlight unit of an LCD device emits light of constant intensity, the display quality of the LCD device may deteriorate depending on the ambient brightness. When the backlight unit emits light of relatively low intensity, images displayed in the LCD device are poorly recognized under high ambient brightness. When the backlight unit emits light of relatively high intensity, power is wasted under low ambient brightness because emitted light of relatively low intensity is sufficient to display recognizable images.